Membrane proteins play vital roles in all living systems. Approximately ˜30% of all genes in almost all sequenced genomes code for membrane proteins. However, our detailed understanding of their structure and function lags far behind that of soluble proteins. As of March 2015, there are over 100,000 structures in the Protein Data Bank. However, there are only 945 membrane protein structures with 530 unique structures including 28 G-protein coupled receptors and no tetraspanin membrane proteins.
There are several bottlenecks in elucidating the structure and function of membrane receptors and their recognition and ligand-binding properties although they are of great interest. The most critical and challenging task is that it is extremely difficult to produce milligram quantities of soluble and stable receptors. Inexpensive large-scale production methods are desperately needed, and have thus been the focus of extensive research. It is only possible to conduct detailed structural studies once these preliminary obstacles have been surmounted.
Zhang et al. (U.S. Pat. No. 8,637,452), incorporated herein by reference, describes an improved process for water solubilizing GPCRs wherein certain hydrophobic amino acids located in the transmembrane regions were substituted by polar amino acids. However, the process is labor-intensive. Further, while the modified transmembrane regions met the water-soluble criteria, improvements in water solubility and ligand binding are desired. Therefore, there is a need in the art for improved methods of studying G-protein coupled receptors.